The present invention relates to multitube double wall bags or sheaths for enclosing the active material of positive plates in electric lead accumulators or batteries.
The invention also relates to the positive plates and batteries provided by the use of the bags and the method for obtaining such bags.
One of the methods for providing positive plates in electric lead batteries is to enclose the active material forming such plates in bags or sheaths of a tubular shape.
It is well known that these bags should present a considerable chemical resistance to the attack of electrolyte (diluted sulphuric acid) and to the attack of nascent oxygen released by the active material surface and should also be provided with a high mechanical strength in order to withstand the changes in volume to which the active material, when converted from PbO.sub.2 to PbSO.sub.4 and vice versa, is subjected during plate operation in the battery.
In order to achieve these results, several designs of bags have been provided, both of single wall and double wall type. Some types of single wall bags have been disclosed, for example, in British Patents No. 1,167,082, No. 809,884 and No. 908,060, in U.S. Pat. No. 2,981,783, 3,462,305 and 2,847,496, and in French Patent No. 2,171,064.
The single wall bags disclosed in these patents are made with fabrics formed of thermorectractable synthetic fibers, or fabrics made of various fibers of organic or/and inorganic nature, particularly synthetic fibers and glass fiber, with the organic fibers interwoven with the inorganic fibers, so as to have externally prevailingly organic ones. In these tubes there is the disadvantage that the synthetic fibers (comprising the mechanically resistant structure of the tubes) do not absolutely insure resistance to electrolyte and oxidation, and can also develop materials contaminating the electrolyte.
In order to overcome this disadvantage, it has been proposed to provide bags formed by double wall tubes, wherein use is usually made of an inner tube formed by fiber glasses and an outer tube comprising a sheet of synthetic material, or made with a synthetic yarn. Particularly, the inner tube is made of chemical grade glass yarn since no other material appears to exhibit such guarantee of chemical resistance, which is mostrly required at contact of the positive active material, just at the limit between solid and liquid phases, where the oxidizing action by nascent oxygen is strongest. It should also be noted that glass, being an inorganic material, is capable of exhibiting the highest guarantee against electrolyte defilement or contamination.
A bag or sheath of this design is shown in French Pat. No. 1, 172,485, the patent disclosing single double wall tubes comprising an inner tube of microporous plastic material and an outer tube of glass fiber or synthetic yarn fabric.
Still another embodiment is described in the U.S. Pat. No. 3,503,807 showing a bag comprising a number of square tubes formed by bending a length of cover material around the four sides of a mandrel. The tubes are stacked and may then be wrapped with a flat sheet of cover material. British Pat. No. 832,569 discloses a multitube bag or sheath, in which each of the individual tubes comprise an outer wall of synthetic fiber and an inner wall of a thin fiber glass felt. the British Pat. No. 844,544 discloses single double wall tubes comprising an inner tube of braided glass yarn and an outer tube formed by a sheet of perforated plastic material. All of the prior art double wall tubes suffer from the disadvantage of being made in a complicated manner and in several stages, since the outer and inner tubes are separately made and only later they are interconnected to produce such double wall tubes. Moreover, single glass yarns present the negative point or feature of having an elongated factor and flexural strength close to nil, They must be braided to be used. Braided tubes can expand, but this is only an apparent elasticity, as any increase in diameter of such tubes would gives a detriment of the length, since glass yarns do not substantially exhibit any elongation.
Attempts have been made to overcome this disadvantage by covering the tubes of braided glass yarn with tubes of plastic material, or to block the same with a painting of thermosetting resin: however, these are but compromise solutions not imparting the required elasticity to the tubes and are unstable and complicated to the highest degree when using glueing sustems to connect the borders or edges of the outer plastic material tube to those of the inner glass yarn tube, or to block the ends of tubes made of glass yarn.
The foregoing is applicable to the case in which the inner glass yarn tubes are first prepared or provided and the outer synthetic material tubes are made thereon.
On the other hand, where double wall tubes are provided by inserting an inner tube in a previously pre-arranged outer tube, several operations are required, and namely the separate forming of the outer tubes, mandrel coating with the inner tubes and inserting such mandrels so coated within the outer tubes. As it will be readily understood, all of these operations are somewhat complicated and laborious and lead to the production of double wall tubes, in which the individual inner and outer tubes, respectively, are loose and independent of one another.
The present applicant has also provided double wall tubes as obtained by overlapping and interconnecting four discrete layers of fabric, the two outer fabrics of which are made of synthetic fibers, whereas the two inner fabrics are of fiber glass, or mixed synthetic and glass fiber, such overlapped layers being interconnected, for example, by seams or heat weldings along parallel lines defining the tubes and separating the same from those adjacent thereto in every multitube bag or sheath. In this embodiment, the disadvantage mainly consists of the necessity of separately providing the four fabrics making up the double wall tubes and then combining and interconnecting such fabrics, as above mentioned.